who is responsible?

Legally, the driver is ALWAYS responsible for the vehicle complying with all legal requirements.

You may have a decent civil case for the company to reimburse you for the fine, but YOU are responsible for driving a vehicle that was in violation.

What DoT inspector gave you a "fix it" ticket for faulty brakes? 99% would have shut you down at the scale until you got the brakes fixed. The company has, I believe, 30 days to correct the defects found and listed on the inspection report then return the form to the state that issued it. I've always gotten clean DoT inspections so I could be wrong about the "30 days". Find somewhere else to work.

Careful, You drove the vehicle, It is your Ticket.

A reputable company would reimburse for the ticket and related expenses, but they do not have to do more than repair their vehicle (or it will be impounded [Red Tagged as inoperable].

As the Driver it is your responsibility to operate a vehicle that is fit for the road. (i.e. You should have refused to  drive, until it was repaired.)

As a professional driver, this should have been  covered in the CDL test and school.

The company should be responsible if you were driving a company vehicle.

you're responsible

The owner of the vehicle is responsible.